Victorius skins are picked based on performance not who is most liked. Think the MVP of champions. Darius Irelia and GP do not qualify as they did not influence the game as much as Maokai. GP had some influence but not so much. Mao hovered around 60-80% pb rate on summer split depending on region so i can see some reason for him.
But i FNCly agree that Mao was far from the top spot. You noticed that i didnt mention Ekko before? Thats because i beleive he should have gotten the skin. He was the champion that got the most influence in the game throught both splits. But after some junglers and supports that are out because we need some diversity in skins, we would have 3 of support/jungler if one got. FYI my picks before ekko would be Nida(almost 100% pb rate both splits, thats insane but got challenger already), Karma (highly contested both splits but she is support) and Rektsai (jungler so out). Elise would be another contester if she already had not got one.
Imo champions that surpassed Maokai's performance this year are:
Top: Ekko Gnar
Jungle: Nida, Rektsai, Elise, Graves
Mid: Victor, Vlad, Ryze
Adc: Ashe, Jhin (mostly for utility, same deal as SIvir)
Supports: Karma by far
There are more but cant think them now and they are close to Mao, but the above surpassed him by a noticable margin. Still without support and jungler i can still see 7 clear picks before him.